The present invention relates to imaging apparatus and method, and more particularly, to an ultrasonic medical imaging catheter.
Conventional medical ultrasonic transducer technology typically has a plurality of pulse driven piezoelectric transducer elements. Phasing techniques are used to drive the transducers to synthesize a one or two dimensional phased array to obtain a high resolution image, such as known in the radar art. This requires that each element in the array be individually driven (pulsed) and have a receive circuit since signals both to and from the transducer elements must be delayed in order to focus the ultrasonic signals. Thus, if there are n number of transducers, n number of interconnect wires and a ground return wire, or a total of n+1 interconnect wires, are required. If it is desired to place the transducers on the tip of a catheter for intralumenal (from the inside) imaging, the number of wires which may be brought back up the catheter is limited by the wire diameter and catheter size. Since a typical minimum number of catheter transducers is about 256, 257 wires would be required for adequate beam forming and thus image resolution. It is desired to reduce this impractically large number to a minimum by implementing an appropriate multiplexing feature.
In U.K. patent no. 8,904,142 issued May 18, 1989 by Kitney et al, there is illustrated a crosswiring arrangement describing a hardwired multiplexing feature. However, the hardwired apparatus appears to multiplex signals on both transmit and receive and would be too lossy to operate as a useful multiplexer for low amplitude signals obtained from low capacitance components. It is an object of the instant invention to overcome these limitations.
In U.S. Pat. No. 4,917,097 issued Apr. 17, 1990 by Proudian et al, a typical synthetic aperture focusing technique is described for transmitting and receiving over one element at a time providing a multiplexing feature which is lossy for low capacitance components. It is therefore an object of the instant invention to overcome this limitation.
In U.S. Pat. No. 4,470,308 issued Sep. 11, 1984 by Hayakawa et al, transducers are directly coupled to a multiplexing means wherein multiplexing is controlled by counters in order to determine variable time delays and compensate for phasing between signals. It is another object of the instant invention to utilize phasing itself as a mechanism to steer and focus the beam.
In U.S. Pat. No. 4,344,159 issued Aug. 10, 1982 by Ballinger and U.S. Pat. No. 4,534,221 issued Aug. 13, 1985 by Fife et al signal isolation is accomplished using isolation diodes. Cross diodes are typically used to isolate low amplitude componenet signals. Ballinger utilizes similar signal isolation then compensates for signal phasing differences. It is yet another object of the instant invention to isolate signals having low amplitude in order to specifically isolate phasing differences; thereby employing phasing itself to steer and focus the beam.
In U.S. Pat. No. 4,911,170 issued Mar. 27, 1990 by Thomas, III et al and assigned to the same assignee as the present invention, a high frequency ultrasonic transducer for characterizing interlumenal arterial features is described. It is still another object of the instant invention to accomodate such high frequency imaging transducers with an improved interlumenal catheter signal acquisition system.
It is therefore an object of the present invention to provide apparatus and method to achieve a sufficiently high resolution image while obtaining a reduction in the number of connecting wires especially when transducers are at a catheter tip.